transmission line

Transmission lines are the kind of thing that seems to confuse beginners. After all, the fact that short-circuits can have infinite impedance and open-circuits can behave like a short is not intuitive at all!. That’s why we like [Tinselkoala]’s latest video that shows a nice model of a transmission line. It helps to understand the line as inductors and capacitors in series-parallel connection.

Any pair of wires used to transmit electrical power have tiny amounts of inductance and capacitance. This is not a problem with DC or low-frequency AC, but when the frequency is sufficiently high, weird things start to happen. The energy tends to escape as radio waves, and current reflects from discontinuities such as connectors and cable joints. For this reason, transmission lines for high frequency signals use specialized construction to minimize those effects and reduce power losses.

[Tinselkoala] has built a model of a transmission line using coils and capacitors to simulate the inductance and capacitance of the line, with LED’s placed between the coils. He feeds the system with the signal generator with frequencies from 10 kHz to 1 MHz. In his words, they act as simple “visual voltmeters” to show the peaks and nodes of the standing waves of voltage in the line.

It is relatively simple to build your own version if you want to experiment with this fascinating subject. You will only need some magnet wire, capacitors, resistors and LED’s. If the subject sounds interesting to you, here you can find an excellent introduction to transmission lines.

Amateur radio has a couple of sweet allocations in the VHF bands, but because the signals don’t reflect off the ionosphere like shortwave signals, the use is limited basically to line-of-sight. One workaround is to use a repeater with a tall antenna, but that requires a lot of infrastructure or a mountainside lair.

What if you could just fly your antenna up in a drone? Well, for starters, you’d run out of batteries pretty quickly unless you could power it remotely. And if you try to tether it, the supply wires end up being too heavy to lift. Or do they?!?!

This great old video (embedded below the break) from Tektronix in the mid-60s covers a topic that seems to confuse folks more than it should — transmission lines. We found it on Paul Carbone’s blog, a great site for aficionados of old analog scopes in its own right.

As with many of these older videos, the pacing is a bit slow by today’s standards, but the quality of the material eventually presented more than makes it worth the effort to reign in your ADHD. For a preview, you can skip to the end where they do a review of all the material.

They start off 5:31 with a pulse travelling down a wire pair, and take a very real-world approach to figuring out the characteristic impedance of the line: if the pulse was created by a battery of 9V, how much current is flowing? If the DC resistance of the wire is zero then there should be an infinite current by Ohm’s law, and that’s clearly not happening. This motivates the standard analysis where you break the wire down into distributed inductance and capacitance.

Of course they do the experiment where you inject a pulse into a long loop of coaxial cable and play around with the termination at the other end of the line. They also measure the velocity factor of the line. Our only gripe is that they don’t tap the line in different places to demonstrate standing waves. The good news is that we’ve got YouTube (and [w3aew]) for that.

If you’ve got 23 minutes to spare, and are curious about transmission lines or just enjoy the soothing voice of a trained radio announcer reading out values of various termination resistors, this old gem is just the ticket. Enjoy!

If it hasn’t been made readily apparent to you by now, power grids are astonishing marvels of technology and quite possibly one of the greatest engineering feats of history. Learning how these systems work is easy in theory, but in practice you will be shot if you try to screw around with at a power station. [Tim] and [Marissa] figured there must be an easier way to learn about power grids so they made their own. It’s small, but it still has everything you’d find in high voltage power lines, minus a hundred kilovolts or so.

This mockup of a power grid simulates a power plant by taking a normal DC motor and connecting that to an alternator and transformer. This is two of the simulated generation points, with the third AC/AC power supply serving as a reference generator for synchronizing phase and frequency. It’s only 12V at 60Hz, but it gets the job done.

A power grid isn’t power plants – there’s also transmission line theory. For this, [Tim] and [Marissa] have a few boards packed with inductors to simulate power lines. There are boards for simulated loads, and synchronization systems built on the MSP430.

In the video below, [Marissa] goes over all the ins and out of the system. It’s very well made and excellent for teaching something that can’t be demonstrated without a practical example.

If you’ve ever wondered why NTSC video is 30 frames and 60 fields a second, it’s because the earliest televisions didn’t have fancy crystal oscillators. The refresh rate of these TVs was controlled by the frequency of the power coming out of the wall. This is the same reason the PAL video standard exists for countries with 50Hz mains power, and considering how inexpensive this method of controlling circuits was the trend continued and was used in clocks as late as the 1980s. [Ch00f] wondered how accurate this 60Hz AC was, so he designed a little test.

Earlier this summer, [Ch00f] bought a 194 discrete transistor clock kit and did an amazing job tearing apart the circuit figuring out how the clock keeps time. Needing a way to graph the frequency of his mains power, [Ch00f] took a small transformer and an LM311 comparator. to out put a 60Hz signal a microcontroller can read.

This circuit was attached to a breadboard containing two microcontrollers, one to keep time with a crystal oscillator, the other to send frequency data over a serial connection to a computer. After a day of collecting data, [Ch00f] had an awesome graph (seen above) documenting how fast or slow the mains frequency was over the course of 24 hours.

The results show the 60Hz coming out of your wall isn’t extremely accurate; if you’re using mains power to calibrate a clock it may lose or gain a few seconds every day. This has to do with the load the power companies see explaining why changes in frequency are much more rapid during the day when load is high.

In the end, all these changes in the frequency of your wall power cancel out. The power companies do the same thing [Ch00f] did and make sure mains power is 60Hz over the long-term, allowing mains-controlled clocks to keep accurate time.